Thermoplastic polymers have become increasingly popular as packaging materials, both as wrapping materials and as containers. This invention has to do with the use of thermoplastic polymers as containers and more specifically, with the application of labels to such containers.
Plastics are widely used as containers, including bottles, jars and jugs (sometimes generically referred to as "containers" hereinafter) for a great number of liquid and granular solid products, thanks to a spectrum of commercially attractive properties. For example, their light weight substantially decreases the shipping weight of the filled container, thereby lowering the cost of shipping the product to the various locations through which the product passes en route to the consumer. Plastics are amenable to the molding of more complex shapes than is glass. Also, losses incurred due to breakage are significantly reduced as compared with those suffered when using glass containers. Plastic containers are also believed to be less of an environmental problem since the polymer can be disposed of by burning if recycling is not available. Accordingly, a very great percentage of the market for liquids, including beverages, food products and liquid detergents, to name just a few, has now been converted to plastic containers.
In recent years, techniques have been developed for applying labels to molded plastic containers prior to ejecting the containers from their mold. This process is referred to as "in-mold labeling" and the labels used therein as "in-mold labels".
In a typical in-mold labeling operation, a plurality of labels carrying various product identification and description information and indicia are die-cut from a master sheet into the required configuration and loaded into a magazine or other type of loading station. For application to a container, the labels are picked from the magazine by an appropriate transfer means, typically vacuum, and inserted into a predetermined label placement area within a plastic forming mold. When molten plastic is charged into the mold for forming into the desired container shape, the molten plastic material contacts the prepositioned label, which is thereupon adhered to the container at the contact area. The contact area is frequently a very slightly raised area on the mold surface which will result in the formation of an indented panel area on the container, in the shape of the label, into said film having a thickness of about 1.75 to 4.0 mils.
In-mold labeling imposes certain conditions on the label stock that are not required of more conventionally applied labels. Specifically, an in-mold label must be stiffer than a label that is applied outside the mold in order that they can be applied to the container by the pick and place technique described in the previous paragraph. If the labels lack sufficient stiffness, they will flex during the transfer operation and, as a result, they will not be uniformly drawn into the mold and what will appear as a wrinkled label will result. Moreover, the heat encountered in the mold may cause the label to soften and an imperfection may be created at the point where the label is seized for transfer to the mold. Still further, label flex during the transfer and application may result in crazing. In order for conventional films to have sufficient stiffness to serve as in-mold labels, the films must be thicker than most conventional films, thus increasing their cost.
Another, and perhaps even more critical, requirement is that the label must undergo a minimum of shrinkage and/or distortion resulting from the heat encountered when it contacts the mold and the freshly molded, and thus still quite hot, container in the mold. Films presently being used as in-mold labels undergo a relatively high degree of heat shrinkage when they are applied in the mold. This is particularly a problem in the case where the container has the indented label placement panel described above. If the label undergoes a high degree of heat shrinkage, it will not fill the label placement panel completely. This is considered an aesthetic flaw in the labeling.
The preferred in-mold label stock in the art today is a multilayer film comprised of a biaxially oriented opaque core layer having a thin layer of an opacified polymer extrusion coated on at least one of its surfaces following longitudinal drawing and prior to transverse drawing. When the structure is thereafter subjected to transverse drawing, the surface layer(s) is (are) only drawn monoaxially. Accordingly, the composite structure is subject to an undesirable degree of heat shrinkage when exposed to the elevated temperatures encountered in in-mold labeling. To minimize this shrinkage, and also to ensure that the film will possess the required stiffness to be handled in the pick and place operation, the film is prepared in a thickness greater than 400 gauge, i.e., greater than 4 mils thickness. Even so, heat shrinkage as great as 5% is frequently observed. When this label is placed in a label placement panel on a container, a clearly perceptible gap results between the label and the outer periphery of the label placement panel.
Thus, presently used in-mold labels are not entirely satisfactory with respect to critical requirements. Additionally, being composites, they have other shortcomings. For example, the composite films usually have very smooth surfaces and, accordingly, air is sometimes trapped between the film and the surface to which it is applied. This can lead to blister or bubble formation between the label and the container, an aesthetic defect. Additionally, composite films are subject to crazing of the surface layers, which leads to another aesthetic defect.
It is the object of this invention to provide a film based on polypropylene that avoids at least the deficiencies cited above and provides an improved stock for use in in-mold labeling of plastic containers.